Small airways and bronchial hyper-responsiveness in cough-variant asthma


 Background

Cough variant asthma (CVA) is one of the special populations of asthma. The study was to compare spirometric parameters of small airways and the degree of bronchial hyper-responsiveness (BHR) between CVA and classic asthma (CA), and examine the relationship between BHR and small airways to determine the accuracy of these markers as indicators of CVA.
Methods

A total of 825 asthmatic patients were screened for the study and 614 were included. All patients performed spirometry and underwent a bronchial challenge with methacholine. It has been estimated that less than 65% of the small airways must be obstructed before changes can be detected using routine pulmonary function tests.
Results

CVA patients showed less small airway dysfunction (< 65%) than those of CA patients with MMEF% predicted (70% vs 80.91%, p = 0.002) and FEF50% predicted (62.71% vs 73.5%, p = 0.004). The function of small airways was higher in the CVA group compared with the CA group (p < 0.001). CVA patients had a mild BHR (p = 0.005). Significant positive correlations were observed between PD20 and MMEF% predicted (r = 0.282, p < 0.001), FEF50% predicted (r = 0.2522, p < 0.001), FEF75% predicted (r = 0.2504, p < 0.001) in patients with CVA. The area under curve of MMEF, FEF50 and FEF75 (% predicted) was 0.615, 0.621, 0.606, respectively. 0.17mcg of PD20 was the best diagnostic value for CVA with an AUC of 0.582 (p = 0.001).
Conclusions

Small airway dysfunction is milder showed in CVA. The value of BHR combined with small airways in CVA prediction, which reflected significant, but not enough to be clinically useful.


Methods
A total of 825 asthmatic patients were screened for the study and 614 were included. All patients performed spirometry and underwent a bronchial challenge with methacholine. It has been estimated that less than 65% of the small airways must be obstructed before changes can be detected using routine pulmonary function tests.

Conclusions
Small airway dysfunction is milder showed in CVA. The value of BHR combined with small airways in CVA prediction, which re ected signi cant, but not enough to be clinically useful.

Background
Asthma is a heterogeneous disorder disease with chronic airway in ammation of bronchial hyperresponsiveness (BHR) to a variety of stimuli, and variable expiratory air ow limitation that is often reversible either spontaneously or as a result of therapy. 1,2 The de nition of "classic asthma (CA)" based on the characteristics and intensity the respiratory symptoms (wheeze, shortness of breath tightness and cough). 1 Page 3/18 The original de nition of cough variant asthma (CVA) was described by Glauser and later by Carrao and McFadden in 1972and 1975, 1979 They described patients with asthma with cough as their sole presenting symptom, but whose symptoms improved with bronchodilators alone. The European and American guidelines do not discuss speci c diagnostic criteria, but they do highlight the diagnostic value of BHR and recommend that the CVA diagnosis should be determined according to the therapeutic response. 1 The 2016 of Chinese Cough guidelines take the detailed diagnostic criteria and treatment of CVA, which is the BHR and successful treatment of bronchodilators and/or inhaled corticosteroids as the basic diagnostic criteria. 6 Small airways were de ned as the bronchial less than 2 mm in internal diameter. 7 They played a role in the pathobiology of asthma and have a distinct role in speci c disease phenotypes, although they are involved in half of all cases of asthma. [8][9][10] The severity of asthma was also associated with in ammatory changes and functional alterations in the small airways. 11,12 The role of the small airways in asthma is increasingly recognized as a potential target in optimal control of the disease. Therefore, this study aims to explore the validity of small airways and BHR in the diagnosis of CVA.

Study design and participants
In this retrospective and observational study, we conducted a series of 614 patients with CVA and CA visiting in The Third People's Hospital of Guangzhou Medical College in Huizhou from January 2018 to April 2019.
CA patients were diagnosed according to a clinical record of wheezing and shortness of breath or/and cough, chest tightness, as well as the presence of BHR or bronchodilator reversibility, based on the 2016 of the Chinese national Guidelines on Diagnosis and Management of Asthma. 13 CVA patients were described chronic cough as their sole or main presenting symptom. The persistent cough was de ned as lasting more than 8 weeks without speci c cause. The cough was usually dry or productive with minimal amounts of clear sputum, and was mainly nocturnal. BHR and responsive to bronchodilators and/or inhaled corticosteroids were the basic diagnostic criteria of CVA. 6 Inclusion criteria were: age greater than 14 years; diagnosis of CA and CVA was made according to the guidelines criteria of China 6, 13 ; received initial diagnosis of CA or CVA and were uncontrolled stage; no other apparent causes of cough were present; not used any oral or/and inhaled corticosteroid (ICS) in the previous 4 weeks.
Exclusion criteria were: a history of chronic obstructive pulmonary disease (COPD) or previous doctordiagnosed asthma-COPD overlap (ACO); a confounding pulmonary comorbidity, such as pulmonary infection and pulmonary tuberculosis; In order to select the CVA, patients with eosinophilic bronchitis (EB), gastroesophageal re ux-related cough (GERC) and upper airway cough syndrome (UACS) were excluded. 6 Also, due to not continuing treatment in order to some reasons or diagnosed as other diseases after treatment, these patients dropped out.

Ethics statement
The study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Ethics Committee of Huizhou third people's Hospital, which absolved the need for written informed consent because of the retrospective study. All personal identi cation data were anonymized and deidenti ed before analysis.

Assessments and spirometry
On the same day the following tests or determinations were performed: spirometry, BHR and induced sputum test. Clinical variables were recorded for the patients.
Spirometry was performed in all patients by pulmonary function instrument (MasterScreen-Pneumo + aps; Jeger, Friedberg, German). Spirometric measurements were made in accordance with recommendations of the Chinese National Guidelines of Pulmonary Function Test. The quality and criteria of spirometry, which characteristics of rapid rise in ow/volume curve, duration of expiration more than or equal to one seconds and visualization of peak expiratory ow (PEF), were required. Repeat at least three times (a variation of no more than 150 ml between two values) and the best was retained. 14 BHR test: The best spirometric value was measured prior to the methacholine challenge. Patient with a percent predicted forced expiratory volume in rst second (FEV1%pred) < 60% was excluded from the BHR test (at baseline). Methacholine solution was prepared at ve different cumulative doses: 0.9% NaCl only, 0.078, 0.312, 1.125 and 2.504 mg in buffered saline. Measure the FEV1 at 60 s to 90 s after inhalation at each cumulative dose from the nebulizer. Obtain an acceptable-quality the largest FEV1 among triplicate measurements at each time for analysis. The procedure was terminated when the FEV1 level fell below 20% of the baseline value. BHR was required to cumulative dose of methacholine that caused fall of FEV1 by 20% of pre-challenge value (PD 20 ), and the positive response was de ned as PD20 ≤ 2.504 mg (between NS and 2.504 mg). The cumulative dose of PD20 was used to assess the degree of BHR. 14,15 Statistical analysis All statistical variables were analyzed using SPSS version 22 (IBM Corporation, Armonk, NY, USA). Data were presented as mean ± standard deviation (SD), frequencies and percentages, as appropriate. A t-test for independent samples or a Chi square test was used to observe two groups of CVA and CA patients. The relationship between PD 20 and spirometry was detected with the Person correlation coe cient.
Correlation between tests was assessed by constructing receiver operating characteristic (ROC) curve. A p value < 0.05 was considered statistically signi cant.

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Characteristics and Pre-challenge spirometry of the patients A total of 825 asthmatic outpatients attended the diagnosis by pulmonary and critical care medicine physicians and underwent a detailed lung function test. In total, 614 asthmatic patients were eligible and analyzed. Reasons for exclusion were 1) aged < 14 years (n = 53) 2)combined diagnosis of COPD or ACO (n = 8 and n = 5, respectively) 3) diagnosis of CA or CVA in the previous (n = 67) 4) performed by bronchodilator reversibility test(n = 78). The strategies of ow-chart is illustrated in Fig. 1.
There were a total of 825 samples with asthmatic patients to be collected in the retrospective study and 211 were included. Demographics paramete of included patients are presented in Table 1. Signi cant but weaker differences were found in sex ratio between CVA and CA patients (p = 0.041).
According to the results of spirometry, FEV1% predicted, FEV1/ forced expiratory vital capacity (FVC) (FEV1/FVC) and PEF% predicted were higher in CVA compared to CA (p < 0.05). We did not nd any difference in age, BMI and history of smoking between the two groups.    Table 3. Signi cantly higher of BHR was found in CA patients than those in CVA patients (p = 0.005). Compared to CVA, CA showed more sensitivity in the degree of FEV1 fall (%) (p = 0.002 and p = 0.001, respectively). Associations between PD20 and FEV1/ FVC, small airway function of the patients Figure 2 gives an overview of all correlations between PD20 and small airways (MMEF%, FEF50%, FEF75%) with CVA and CA. Signi cant positive correlations were observed for MMEF% predicted (r = 0.282, p < 0.001), FEF50% predicted (r = 0.2522, p < 0.001) and FEF75% predicted (r = 0.2504, p < 0.001) in patients with CVA (Fig. 2a). We also found signi cant correlations between PD20 and small airways (r = 0.2861, r = 0.2917 and r = 0.2476, respectively) with CA patients (p < 0.001) (Fig. 2b).
The ROC curve of Table 4 and Fig. 3 presented PD 20 and proportion parameters in spirometry as predictors to identi ed CVA from CA. The optimum cut-point for FEV1%predicted was 82.65% with an area under the curve (AUC) of 0.602 (p < 0.001), and the AUC of MMEF, FEF50 and FEF75 (% predicted) was 0.615, 0.621, 0.606, respectively. In addition, 0.17mcg of PD20 was the best diagnostic value for CVA with an AUC of 0.582 (p = 0.001). Data of sensitivity and speci city were showed in Table 4. The AUC of PD20 combined with MMEF (% predicted) was 0.616, and that combined with MEF50 (% predicted) and MEF75 (% predicted) was 0.625, 0.606, respectively (Fig. 4).

Discussion
The retrospective study showed that more patients with CVA were associated with differences in levels of sex, FEV1% predicted, FEV 1 / FVC, compared to patients with CA. Milder BHR and small airway dysfunction were showed in CVA, and the correlation between them were observed weak but signi cant differences. BHR and small airway were poor to distinguish patients with CVA, respectively. We had also evaluated the value of BHR combined with small airways in CVA prediction, which re ected signi cant, but not enough to be clinically useful.
Chronic cough is the sole presenting symptom of CVA. Previous literatures have studied that chronic persistent non-productive cough is more frequent in females, and females are more easily troubled by the symptom. 16,17 Females exceed males in number among patients attending specialist respiratory clinics. 18 The cough threshold is lower in females than in males, illustrating that the cough sensitivity is heightened in females. 19,20 In our study, the difference of gender is weak. The numbers of female was a little frequent in CVA (p = 0.041). The mechanism of the gender differences is still unclear, and additional studies are needed to better understand gender differences.
Spirometry is the fundamental diagnostic method and the most widely non-invasive, easy to assess the air ow limitation associated with asthma. Parameters such as FEV1 and PEF are frequently used to evaluate proximal airway obstruction. In the study, we found that patients with CVA were more likely to have a better FEV1 and FEV1%predicted (89.21% vs 85.02, p 0.001). Spirometric values indices are almost independent of patient's activity if the expiration is forced; they depend only on the properties of the respiratory system because of the air ow limitation phenomenon. 21,22,23 Evaluation of forced spirometry results begins with analysis whether bronchial air ow capacity quanti ed by means of the FEV 1 .
Previously, asthma was understood to be a disease primarily of the central airways. However, surgical lung specimens with living chronic asthma and autopsy specimens with fatal asthma reveal mucus plugging and in ammatory involvement of both the small and large airways. 24,25 An in ammatory characterized by increased T cells, activated eosinophils and major basic protein in the small airways, which was similar to the in ammation of the central airways. 25 The intensity of the in ammation may be even higher in the small airways compared with central airways. 26 These observation con rm that the chronic in ammation of asthma involves the entire lung, from the large proximal to the small distal airways. Assessment of small airways pathology can be subdivided in tests measuring ow, airway resistance, inhomogeneity of ventilation distribution and hyperin ation or air trapping by pulmonary function machines. Flow measures of small airways commonly used in forced expiratory ow at 50% (FEF50%), at 75% (FEF75%) and at 25-75% (FEF25-75%/MMEF) of FVC. Among these parameters, FEF 25 − 75% is the most commonly adopted, although the literature supporting its reliability is not conclusive.
The pathophysiological features of CVA are similar to those of CA. CVA shows similar levels of eosinophilic airway in ammation and a milder degree of airway remodeling, such as sub-epithelial thickening, goblet cell hyperplasia, and vascular proliferation. 27,28,29 The maximal airway response on the dose-response curve is clinically relevant component of BHR because it re ects the potential degree of airway obstruction [30]. BHR of CVA is caused by the type of airway in ammation and airway structural changes, which show similar to CV. 28,29,30,31 Nearly 30% of CVA patients eventually develop to CA, sometimes severe enough to require continuous treatment. 5,32 Some researches indicate that increasing BHR has a pathogenetic role in the development of wheezing during the course of CVA on exposure to an allergic or a non-allergic stimulus. 33,34 Given these studies, CVA is considered to be the initial stage of asthma. 34 Some patients with CVA will evolve into continuous CA. Therefore, the early diagnosis and treatment is recommended to attenuate the in ammation and remodeling. In our study, the spirometric parameters for small airways (FEF50%, FEF75% and MMEF) with CVA group were higher than those with CA group. Small airway dysfunction was present in a large proportion of asthma patients at baseline, and less in CVA compared to those of CA. Our study has provided a positive relationship between PD20 and small airways in both CVA and CA.

Limitation Of The Study
Our study reported on a clinical study with 825 patients with asthma to investigate the diagnostic validity of small airways and BHR in cough-variant asthma. A positive relationship showed between PD20 and small airways. However, overall the association appeared weak with low AUCs for the prediction of BHR and small airways to CVA. While these correlations might show statistical signi cance, none of these appear convincing and potentially clinically relevant judged. Also, the low AUC values are very unreliable and, very likely, not helpful to inform clinical decision making. Statistically signi cant does not always imply clinical signi cance, but the level of association needs to be considered, as well.

Conclusions
This study provides evidence that small airway obstruction and BHR are milder in patients with CVA, and the relationship of them is weak. Small airways and BHR may be used to detect CVA patients. Based on these weak correlations and poor prediction values, further investigations would be required.

Declarations
Ethics approval and consent to participate: The study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Ethics Committee of Huizhou third people's Hospital, which absolved the need for written informed consent because of the retrospective study. All personal identi cation data were anonymized and de-identi ed before analysis.
Consent for publication: Not applicable.